The Ce ln a Basis of Reproclu on and nher tance

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1 The Ce ln a Basis of Reproclu on and nher tance Focus on the Concepts Reproduction and in_heri.tance are fundamental characteristics o life, and the process of cell division is essential to reproduction and inheritance. As you study the chapter, focus on the following concepts: Some organisms can reproduce asexually, duplicating their chromosomes and then undergoing cell division to produce identical offspring. In sexual reproduction, offspring inherit a mix of genes and chromosomes from two parents, so they are not identical to their parents or each other. Cell division takes place in the context of the ceil cycle. Between divisions, a cell replicates its DNA. At the start of division, each chromosome consists of a pair of chromatids--the two duplicate DNAs. During mitosis, the chromosomes line up and the chromatids separate. The cell sp.lits in a process called cytokinesis, producing two daughter cells identical to the original. Mitosis provides for growth, cell replacement, and asexual reproduction. Cells usually do not divide unless signaled by chemical growth factors that control the cell cycle. Most animal cells must be in contact with a surface to divide, and will stop dividing when crowded. Cancer cells do not respond to these ceil cycle controls; they divide excessively and can invade other tissues. Ceils of each species contain a characteristic nu_rnber of chromosomes. In humans, somatic (body) cells are diploid, containing two sets of 23 chromosomes m 23 from the mother and 23 from the father--for a total of 46. The two chromosomes of a matching pair are called homologous chromosomes. A process called meiosis produces haploid eggs and sperm (gametes)--each containing one set of 23 chromosomes. Egg and sperm join in fertilization to produce a diploid zygote with 46 chromosomes, which divides repeatedly to grow into an adult. In diploid organisms, meiosis produces haploid gametes, in a process that involves two consecutive cell divisions. In the first division, unlike mitosis, homologous pairs of chromosomes line up and then the pairs separate, producing two haploid daughter cells. In the second division, the sister chromatids of each chromosome separate in each cell, producing a totalof four haploid cells. Each chromosome carries genes for certain traits, and homologous chromosomes can carry different versions of genes. The mixing of chromosomes that occurs during meiosis and fertilization can generate a variety of offspring in three ways: First, homologous chromosomes line up independently in meiosis, creating many possible sets of 23 when the pairs separate. Homologous chromosomes cross over and exchange segments while they are paired, generating greater variety. With such a diversity of eggs and sperm, fertilization produces further variation among offspring.

2 Chapter 8 Alterations in chromosome structure, such as duplications or deletions, can cause birth defects and cancer. Errors in ffteiosis can produce gametes--and. offspring--with extra or missing chromosomes. In humans, an abnormal number of chromosomes can cause birth defects such as Down syndrome, although consequences of missing or extra chromosomes are usually more serious or fatal. Sometimes an error in cell division can produce gametes and offspring with extra sets of chromosomes. Many species of plants and some animal species appear to have originated this way. Review the Concepts Work through the following exercises to review the concepts in this chapter. For additional review, refer to the activities at The website offers a pre-test that will help you plan your studies. Review the concepts introduced in these modules by filling in the blanks. "Like begets 1." This old saying means offspring look like their parents. Technicall)~ only offspring produced by 2 reproduction look exactly like their parents, because they inherit all their 3 from a single parent. For example, when an amoeba divides, its 4 is duplicated, and identical sets of 5 (the structure~ that contain most of the amoeba s DNA) are allocated to opposite sides of the ceil. The parent amoeba splits, and the two daughter amoebas that are formed are genetically 6 to each other and to the 7 ceil. This kind of cell division, called 8, is also seen in multicellular organisms. A single fertilized egg 9 into a multicellular body via mitosis. Mitosis enables an adult animal or plant to 10 lost or damaged cells. Some multicellular organisms, such as sea stars, can 11 asexually from buds or fragments via mitosis. Prokaryotes also reproduce asexually, via a type of cell division called binary 12. Most genes in a prokaryote are carried on a single 13 DNA molecule, which is much 14 than the multiple chromosomes of eukaryotes. The prokaryote starts by 15 its DNA. The cell elongates, and the duplicate chromosomes become separated. Final176 the plasma 16 grows inward and splits the cell in two. Like mitosis ift an amoeba, binary fission produces two 17 cells identical to the original parent cell. The offspring produced by sexual reproduction resemble their parents, but they are not identical to their parents or to each other. Sexual reproduction begins with the production of an 18 and a 19 o specialized cells that join to produce an offspring. These cells are formed only in reproductive organs via a special kind of cell division called 20. Each egg or sperm carries only 21 as many chromosomes as the parent cell, and each has a unique combination of genes. When an egg and sperm fuse, a fertilized egg inherits a particular set of genes from each parent, and through repeated cell divisions, it develops into an organism with a unique combination of traits. Thus, sexual, reproduction produces among offspring. Via sexual reproduction "like begets like," but not exactl)a

3 The Celhdar Basis of Reproduction and Inheritance 93 Test your knowledge of chromosomes by completing this crossword puzzle. ~ 3 act oss 4. cells have more chromosomes and genes than prokaryotes do. 5. A typical has about 3,000 genes. 6. Chromatin strands are too thin to be seen under a microscope. 7. Chromosome duplication produces two sister 9. A chromosome contains one long molecule. 10. Chromatin strands are much longer and than chromosomes. 11. Eukaryotic chromosomes involve more molecules than those of bacteria. 13. When a chromosome divides, one sister chromatid goes to each cell. ~4. Proteins help determine chromosome structure and control activity. Down 1. Sister chromatids are joined at the 2. The number of chromosomes in a eukaryotic cell depends on the 3. When a cell is not dividing, chromosomes form long, tl~n fibers called 8. Genes in eukaryotic cells are grouped into chromosomes. 9. Chromosomes are clearly visible only when a cell is 12. cells carry about 20,000 genes in 46 chromosomes.

4 Chapter 8 Review the cell cycle: First identify the parts of the cycle and place them in order by writing the name of each phase or process on the diagram. Choose from: S, interphase, mitosis, G~, mitotic phase, cytokinesis, and G2. Then add abrief description of what is happening. during that portion of the cycle. Choose from: Growth and DNA synthesis, growth and activity following division, division of cytoplasm, growth and activity between divisions, division of nucleus and chromosomes, growth and activity between DNA replication and division, and mitosis plus cytokinesis. Description: Description: Description: Description:

5 The Celhdar Basis of Reproduction and Inheritance Summarize mitotic cell division. Briefly describe the appearance and activities of each of these cell parts during interphase and the five stages of mitosis. Include a simple sketch for each phase (just cell outlines and chromosomes). Nucleus and nuclear envelope Mitotic spindle Interphase Prophase Prometaphase Metaphase Anaphase Telophase Chromosomes Ceil size and shape Sketch This is a photograph of cells in an onion root tip, an area of rapid ceil division. In which stage of mitosis (or interphase) is each of the numbered cells?

6 Chapter 8 Match each structure on the left with its correct role in mitosis in an animal cell on the right. ~ 1. Metaphase plate 2. Kinetochores 3. Sister chromatids 4. Microtubules ~ 5. Centrosome 6. Motor proteins A. Where spindle microtubules attach to chromosomes B. Extend from pole to pole and help move chromosomes C. Separate at the begirming of anaphase D. Material around centrioles from which mitotic spindle grows E. "Walk" chromosomes along microtubules toward cell poles F. Chromosomes come to rest here during metaphase Read this module and then write a statement containing exactly 30 words (no more, no less!) comparing cytokinesis in plant and animal ceils. (Writing exactly 30 words will help you to think about the processes and choose your words carefully. It s fun to try it.) Review the functions of cell division and the factors that control it by filling in the blanks below. Mitotic cell division has several important functions. Some organisms rely on cell division for 1 reproduction. Hydra, for example, produces buds that detach from the parent and take up life on their own. Cell division is also responsible for 2 of human embryos and plant roots. In an adult human, some cells, such as most nerve and muscle cells, cease to divide. Others, such as cells of the 3, divide only if the organ is damaged. This process 4 wounds. Some cells, such blood cells and cells on the surface of the 5, are constantly being damaged or lost. These cells are 6 by cell division. In each of these cases, the new cells have exactly the same 7 and 8 of chromosomes as the parent cells because of the way duplicated chromosomes are distributed in the process of 9 Growth, cell replacement, and reproduction require control of the rate and timing of cell division. Much has been learned by studying cells grown in laboratory 10. Ceils growing in a laboratory dish will divide only when in contact with a solid 11. In the body, this 12 dependence may keep normal cells from dividing if separated from their normal surroundings. Cells will multiply only until they touch one another, a phenomenon known as 13 Apparently, cells rely on proteins called 14 to tell them when to divide, and will stop when cells are crowded and these substances are depleted. There are over fifty known growth factors with various ftmctions. Vascular endothelial growth factor is one example; it stimulates blood 15 growth.

7 The Celhdar Basis of Reproduction and Inheritance It appears that growth factors influence cell division by acting on the cell cycle 16 system, a set of molecules that triggers and coordinates events in the cell cycle. The system automatically 17 cell division at several major checkpoints unless the "brakes" are overridden by go-ahead signals. There are checkpoints in the G 2 and M phases of the cell cycle, but the most important checkpoint for many cells is the 18 checkpoint. If a cell receives a go-ahead signal in the form of a growth factor at the G1 checkpoint, the cell will proceed into the phase of the cell cycle, replicate its DNA, and eventually divide. A growth factor probably acts on a cell by attaching to a 2o protein in the cell membrane. This protein in turn generates a signal that acts on the cell cycle control system within the cell. In the absence of a go-ahead signal, a cell will cease dividing. Many of our cells that can no longer divide 21 cells, for example--are stopped at the G1 checkpoint. Sometimes ceils escape these control mechanisms, divide uncontrollably, and invade other body tissues. 22 cells can ldll the organism. In cell culture, they can grow without being attached to a solid surface, are unaffected by density-dependent inhibition, and are less affected than normal cells by growth factors and 23 signals. Cancer cells can go on dividing indefinitely (unlike normal cells, which can divide in culture for only 24 generations). A mass of cancer cells is called a 25. If the cells of a tumor remain at the.original site, the tumor is called a 26 tumor; if the cells spread, it is a 27 tumor. The spread of cancer cells is called 28 Carcinomas are cancers that arise in body coverings, such as the skin. 29 arise in support.tissues, such as muscle or bone. 3o are cancers of bloodforming tissues. Cancer treatments, such as 31 and 32 ~ slow cancer by interfering with 33. The anticancer drugs vinblastin and taxol prevent cell division by disrupting the mitotic 34 Describe the relationship between the terms or items in each of the following pairs. 1. Sex chromosomes and autosomes 2. The two chromosomes of a homologous pair 3. The two sister chromatids of a single chromosome 4. A diploid cell and a haploid cell 5. A somatic cell and a gamete 6. An egg and a zygote 7. Fertilization and meiosis 8. Mitosis and meiosis 9. X and Y chromosomes 10. Gene and locus 11. Life cycle and cell cycle

8 Chapter 8 Review meiosis by drawing in the chromosomes to complete this sequence of diagrams. Some have been done for you. Label meiosis I, meiosis II, the phases of meiosis I and U, a pair of homologous chromosomes, two sister chromatids, and an example of crossing over. 10. and 11. and

9 The Celhdar Basis oj: Reproduction and Inheritance Compare mitosis and meiosis by completing this chart. Mitosis Meiosis 1. Produces haploid daughter cells unlike parent cell Involves one cell division 2. Produces two daughter cells Homologous chromosomes pair and then separate Individual chromosomes line up 5. at metaphase plate No crossing over occurs Needed for sexual reproduction These modules discuss how independent orientation of chromosomes, random fertilization, and crossing over can lead to varied offspring. The diagram following shows the two homologous pairs ol chromosomes in a cell with a diploid number of 4. Three different genes are also shown. On separate paper, complete the sketches described in questions 1 through 3. o Show how two different orientations of the chromosomes during metaphase I of meiosis could lead to four different combinations of genes in gametes (assuming crossing over does not occur). (You don t need to show meiosis step by step--just the outcome.) Show how crossing over could recombine genes on the larger pair of chromosomes, producing different gametes. How many different combinations of genes in gametes are possible if these two processes happen simultaneously? Try to sketch all of them.

10 Chapter 8 A karyotype is an inventory of chromosomes. Errors in meiosis can produce gametes with. extra or missing chromosomes. Fertilization of these abnormal gametes produces offspring with genetic abnormalities. Sometimes new species arise from chromosomal errors. Review karyotyping and alterations in chromosome number by completing this crossword puzzle.

11 The Cellular Basis of Reproduction and Inheritance Across 1. An abnormal number of chromosomes often results in 4. A woman with only one X chromosome is said to have syndrome. 5. The Y chromosome carries genes than the X chromosome. 7. The incidence of Down syndrome increases with the of the mother. 8. Down syndrome individuals are prone to ~ and Alzheimer s disease. 9. A single Y chromosome is enough to produce 12. Nondisjunction results in extra or missing chromosomes in 16. ~ is when members of a pair of chromosomes fail to separate during meiosis. 18. Most human offspring with abnormal numbers of chromosomes are spontaneously 21. Down syndrome is the most serious common defect in the United States. 23. Pregnant women over 35 years of age are candidates for ~ testing. 26. In meiosis II, ~ chromatids may fail to separate. 27. A is a photographic inventory of an individual s chromosomes. 28. A pla_nt whose cells have four sets of chromosomes is said to be ~ Down 2. A person with Down syndrome has ~ number 21 chromosomes. 3. who are lacking an X chromosome are desigriated XO. 4. XXY or XXYY males have small and feminine body contours. 6. In meiosis I, a pair of chromosomes may fail to separate. 10. If a sperm fertilizes an egg with an extra chromosome, the will have an extra chromosome. 11. Nondisjunction can alter the number of chromosomes, as well as autosomes. 13. Chromosomal defects usually result from errors in ~ 14. Individuals with Down syndrome are usually mentally 15. The gray tree is a tetraploid animal. 17. People with Down syndrome live -than-normal life spans. 19. An extra chromosome 21 is called A person with an extra number 21 chromosome has syndrome. 22. of the eggs of a Down syndrome woman will have an extra chromosome. 24. An unusual number of sex chromosomes has effect than an unusual number of autosomes. 25. Many new ~ of plants seem to have arisen from errors in cell division.

12 Chapter 8 Chromosomes sometimes break ~eir parts can become scrambled, and abnormalities can result. Match each of the diagrams of chromosome alterations with its name (A-D) and a description of its effects (W-Z). Diagram Name Effects 2. o Names: A. Deletion B. Duplication C. Inversion D. Translocation Effects: W. X. Z. May cause chronic myelogenous leukemia in somatic cells Least likely to have serious effects, because genes are still present in normal numbers Likely to have the most serious effects, as in cri du chat syndrome A chromosome fragment breaks off and joins a homologous chromosome

13 The Cellular Basis of Reproduction and Inheritance Test Your Knowledge Multiple Choice 1. There are a number of differences between binary fission of a bacterium and human cell division. Which of the following is not one of them? a. A bacterium has only one chromosome. b. Human cells undergo mitosis and cytokinesis. c. Bacteria are smaller and simpler than human cells. d. Bacteria have to duplicate their DNA before dividing; human cells do not. e. Human chromosomes are larger and more complex. 2. You would be unlikely to see which of the following human cells dividing? a. muscle cell b. skin cell c. cancer cell d. cell from an embryo e. bone marrow cell 3. Which of the following correctly matches a phase of the cell cycle with its description? a. M--replication of DNA b. S--immediately precedes cell division c. G2--cell division d. Gl--immediately follows cell division e. All of the above are correctly matched. 4. Which of the following is not true of human somatic cells? a. They arise by mitotic cell division. b. They are haploid. c. They are body cells other than eggs and sperm. d. They are larger and more complex than bacterial cells. e. They contain 46 chromosomes. 5. In telophase of mitosis, the mitotic spindle breaks down and nuclear envelopes form. This is essentially the opposite of what happens in a. prophase. b. interphase. c. metaphase. d. S phase. e. anaphase. 6. Sister chromatids a. cross over during prophase I of meiosis. b. separate during the first meiotic division. c. are produced during S phase between cell divisions d. cross over during prophase II of meiosis. e. are also called homologous chromosomes. Which of the following is not a function of mitotic cell division in animals? a. asexual reproduction b. growth c. repair of damaged organs d. production of gametes e. cell replacement Meiosis a. is responsible for body growth and repair. b. halves the number of chromosomes in cells. c. is the process by which the body produces diploid cells. d. follows mitosis and splits the cytoplasm in two. e. is important in asexual reproduction. Crossing over is a. important in genetic recombination. b. what makes a cell become cancerous. c. a key process that occurs during mitosis. d. an important mechanism of chromosome repair. e. what prevents cells from multiplying indefinitely in cell culture. Human are diploid, and human are haploid. a. sex chromosomes.., autosomes b. autosomes.., sex chromosomes c. somatic cells.., gametes d. gametes.., somatic cells e. chromosomes.., chromatids Which of the following does not lead to genetic variability? a. random fertilization b. crossing over during meiosis c. division of chromosomes during anaphase of mitosis d. orientation of chromosomes during metaphase I of meiosis e. genetic recombination Most cells will divide if they receive the proper signal at a checkpoint in the ~ phase of the cell cycle. a.m b. G1 c.s d. G 2 e. cytokinesis

14 104 Chapter 8 Geneticists suspect that the extra chromosome seen in Down syndrome usually comes from the egg, rather than the sperm, because a. eggs are produced so rapidly that there is more chance for error. b. Down syndrome is due to a dominant gene in women, a recessive gene in men. c. most women inherit Down syndrome from their mothers. d. eggs are produced in much larger numbers than sperm. e. the probability of Down syndrome increases with the age of the mother. 14. Which of the following chromosomal alterations would you expect to have the most drastic consequences? a. inversion b. duplication c. translocation d. deletion e. a and b are equally the most serious 15. Disorders involving unusual numbers of sex chromosomes show that "maleness" is caused by the a. presence of an X chromosome. b. presence of a Y chromosome. c. absence of an X chromosome. d. absence of a Y chromosome. e. absence of an X chromosome and presence of a Y chromosome. Essay 1. Explain why, strictly speaking, the phrase "like begets like" applies only to asexual reproduction. 2. Briefly describe mitosis and cytokinesis and state their functions. Describe how cancer cells differ from normal body cells. Compare mitosis and meiosis. What are their functions? Which produces haploid cells? Diploid cells? What kinds of cells undergo mitosis and meiosis? What kinds of cells are produced by each? How many cells are produced? How might the genes on the sister chromatids of a certain chromosome compare with each other and with the genes on the sister chromatids of the homologous chromosome? Describe three aspects of sexual reproduction that lead to the production of varied offspring. o Explain how an error in meiosis can cause a baby to be born with an extra or missing chromosome. About one in 700 babies born in the United States possesses an extra chromosome 21, resulting in Down syndrome. Why are few individuals seen who have extra copies of other chromosomes? Apply the Concepts Multiple Choice 1. In certain fungi and algae, cells undergo mitosis repeatedly without subsequently undergoing cytokinesis. What would result from this? a. a decrease in chromosome number b. inability to duplicate DNA c. division of the organism into many cells, most lacking nuclei d. large cells containing many nuclei e. a rapid rate of sexual reproduction 2. A human bone marrow cell, in prophase of mitosis, contains 46 chromosomes. How many chromatids does it contain altogether? a. 46 b. 92 c. 23 d. 23 or 46, depending on when during prophase you look e. 46 or 92, depending on when during prophase you look Which of the following is the most significant difference between mitosis and meiosis? a. Chromosomes are duplicated before mitosis. b. Meiosis is not followed by cytokinesis. c. Homologous pairs of chromosomes are split up in meiosis. d. A spindle formed of microtubules moves the chromosomes in mitosis. e. Crossing over occurs in mitosis. If there are 22 chromosomes in the nucleus of a toad skin cell, a toad egg woul d contain ~ chromosomes. a. 22 b. 44 c. II d. 33 e. 88

15 The Cellular Basis of Reproduction and Inheritance o 10. Which of the following carry the same genetic information? a. sister chromatids b. X and Y chromosomes c. all autosomes d. homologous chromosomes e. all haploid cells A cell biologist carefully measured the quantity of DNA in grasshopper skin cells undergoing mitosis in cell culture. Cells examined during the G2 phase of the cell cycle contained 200 units of DNA. What would be the amount of DNA in one of the grasshopper daughter cells seen in telophase of mitosis? a. 50 units b. 100 units c. between 50 and 100 units d. 200 units e. 400 units What would be the quantity of DNA in one of the grasshopper cells (question 6) produced by telophase II of meiosis? a. 50 units b. 100 units c. between 50 and 100 units d. 200 units e. 400 units The two chromosomes of a homologous pair a. carry identical genetic information at corresponding locations. b. carry information for the same characteristics at different locations. c. carry identical genetic information at different locations. d. carry information for the same characteristics at corresponding locations. e. Any of the above is possible. A picture of a dividing pigeon cell taken through a microscope shows that the cell contains seven chromosomes, each consisting of two chromatids. This picture must have been taken during a. metaph~se of mitosis. b. prophase I of meiosis. c. telophase II of meiosis. d. prophase II of meiosis. e. telophase of mitosis. A culture of mouse cells is treated with a chemical that interferes with the activity of microfilaments. Which of the following will probably be affected the most? a. mitosis b. chromosome duplication c. pairing of homologous chromosomes d. cytokinesis e. joining of sister chromatids at the centromere A zoologist examined an intestine cell from a crayfish and counted 200 chromosomes, each consisting of two chromatids, at prophase I of mitosis. What would he expect to see in each of the four cells at telophase II of meiosis if he looked in the crayfish ovary? a. 50 chromosomes, each consisting of two chromatids b. 50 chromosomes, each consisting of one chromatid c. 100 chromosomes, each consisting of two chromatids d. 100 chromosomes, each consisting of one chromatid e. 200 chromosomes, each consisting of one chromatid One chromosome of a homologous pair carries the genes J and K. The other chromo- " some of the pair carries the genes j and k at corresponding loci. Crossing over results in exchange of chromosome segments and production of gametes with new combinations of genes. A "recombinant -type gamete resulting from this crossover might contain a. genes J and K. b. genes j and K. c. genes J and j. d. genes j and k. e. genes K and k. Humans have 23 pairs of chromosomes, while our Closest relatives, chimpanzees, have 24. Chromosome studies indicate that at some point early in human evolution, two chromosomes simultaneously broke into a large portion and a small portion. The large parts combined to form a large chromosome, and the small parts combined to form a much smaller chromosome, which was subsequently lost. This important chromosomal change could best be described as a. nondisjunction followed by deletion. b. translocation followed by deletion. c. duplication followed by deletion. d. translocation followed by inversion. e. nondisjunction followed by inversion.

16 106 Chapter It would be most difficult to spot which of the following by looking at a karyotype? a. an extra chromosome b. part of a chromosome duplicated c. a missing chromosome d. par t of a chromosome inverted e. a translocation Essay 1_., A fruit fly muscle cell contains 8 chromosomes. When a fruit fly ovary cell undergoes meiosis, independent orientation of chromosomes will produce how many different combinations of chromosomes in the fly s eggs? a. 4 b. 8 c. 16 d. 64 e. 256 Recall how chemotherapy slows the growth of cancerous tumors. Common side effects of chemotherapy are hair loss, nausea, loss of appetite, and anemia (drop in red blood cell count). Why do you think that chemotherapy has some of its strongest side effects on the skin, the lining of the digestive tract, and bone marrow (where blood is made)? Most of the grasshopper somatic cells examined by the cell biologist in Multiple Choice questions 6 and 7 contained either 100 or 200 units of DNA. Some of the interphase cells, however, contained between 100 and 200 units. Explain what was happening in these cells. A slide of dividing cells in an onion root tip (Figure 8.10A in the text) is a "snapshot" in time. Each cell is stopped at the particular point in its ceil cycle when the slide was ~ made. A biology student examined such a slide under a microscope. Out of 100 cells she caught in the act of dividing, 38 were in prophase, 15 in prometaphase, 8 in metaphase, 10 in anaphase, and 29 in telophase. Assuming that the cells are growing and dividing independently, what do these data tell you about the phases of mitosis in onion cells? A white blood cell from a female golden retriever was found to contain a total of 78 chromosomes. How many different kinds (sizes and shapes) of chromosomes would you expect to find in the cell? Why? The somatic cells of a mosquito contain three pairs of chromosomes--two large ones, two medium-sized ones, and two small ones. One large chromosome bears the A gene; its homologue bears the a gene. One mediumsized chromosome bears the B gene; its homologue bears the b gene. One small chromosome bears the C gene; its homologue bears the c gene. Sketch cells at metaphase I of meiosis in the ovary of the mosquit O, showing the different alignments of chromosomes that are possible: Then show how these lead to different combinations of genes in the gametes. How many different combinations are possible in the eggs? In the sperm of a male mosquito with the same genes, how many different gene combinations are possible? If the male and female mate and her eggs are fertilized with his sperm, how many different combinations are possible in the zygotes? One chromosome of a homologous pair carries genes Q and R. Its homologue carries genes q and r. Show how crossing over during meiosis could produce gametes with new combinations of genes. What combinations of genes occur in parental-type gametes? In recombinant-type gametes? Which of the following abnormalities in sex chromosomes would result in a predominantly male phenotype? A predominantly female phenotype? Explain why. XXXX, XXY, XYYY, X,

17 The Cellular Basis of Reproduction and Inheritance Put Words to Work Correctly use as many of the following words as possible when reading, talking, and writing about biology: anaphase, anchorage dependence, asexual reproduction, autosome, benign tumor, bina~d jlssion, carcinoma, celt cycle, eeli cycle control system, ceil division, eeli plate, centromere, centrosome, checkpoint, chemotherapy, chiasma (plural, chiasmata), chromatin, chromosome, cleavage, cleavage furrow, crossing over, cytokinesis, daughter cell, deletion, density-dependent inhibition, diploid cell, Down syndrome, duplication,~fertilization, G1 phase, G2 phase, gamete, genetic recombination, genome, growth factor, haploid cell homologous chromosomes, interphase, inversion, lcaryohype, kinetechore, leukemia, life cycle, locus (plural, loci), lymphoma, malignant tumor, meiosis, meiosis L meiosis II, metastasis, mitotic phase (M phase), mitotic spindle, mitosis, motor protein, nondisjunction, parent cell prometaphase, prophase, S phase, sarcoma, sex chromosome, sexual reproduction, sister chromatid, somatic cell, synapsis, telophase, tetrad, translocation, trisomy 2I, tumor, zygote Use the Web Cell division is another of those processes best understood ff seen in motion. See cell division activities, videos, and animations at

18 Patterns of inheritance Focus on the Concepts The characteristics of organisms are shaped by genes inherited from their parents. A few fundamental rules underlie patterns of inheritance. In studying this chapter, focus on the following concepts: Discrete factors called genes govern inheritance. An organism s phenotype (characters and traits) is shaped by genotype (genes and alleles). A sexually reproducing organism inherits two alleles for each character, one from each parent. These alleles occur on homologous chromosomes and may be the same or different. Homologous chromosomes, and their allele pairs, segregate from one another during the production of gametes. An egg or sperm carries only one allele for each character. Fertilization results in offspring with various combinations of allele pairs. The Punnett square predicts combinations of alleles in gametes and offspring, and ratios of resulting offspring phenotypes: The F2 of a monohybrid cross shows a 3:1 phenotypic ratio. Each pair of chromosomes, and each pair of alleles, separate independently of other pairs during meiosis. The Punnett square can also track two different characters at. once, predicting combinations of alleles in gametes and offspring, and ratios among offspring phen0types. The F 2 of a dihybrid cross shows a 9:3:3:1 phenotypic ratio. Rules of probability can be used to predict patterns of inheritance. The probability of a compound event is the product of separate probabilities of independent events. The probability of an event that can occur two different ways is the sum of the probabilities of the different ways. Many inherited human disorders are caused by a single gene. Many of these disorders are recessive and occur in offspring,of heterozygous carrier parents. There are more complex patterns of inheritance: Sometimes there are more than two alleles for a character, and sometimes alleles are not clearly dominant or recessive. Sometimes one gene shapes many characters, and sometimes one character is shaped by many genes. Linked genes (those on the same chromosome) do not assort independent!35 confounding Mendel s ratios but allowing us to map chromosomes. In many species, chromosomes determine sex. In humans, sex is determined by the X and Y chromosomes. Females are XX, and males are XY. The SRY gene on the Y.produces a male. Sex-linl<ed genes (almost always on the X) exhibit unique patterns of inheritance; males are more likely to exhibit sex-linked recessive defects. i08